The invention relates to a transmission system for transmitting datawords via a multicarrier signal from a transmitter to a receiver, the transmitter comprising a generator for generating for each dataword a number of alternative sequences, and the transmitter further comprising a selector for selecting the alternative sequence with the lowest peak power value for transmission to the receiver.
The invention further relates to a transmitter for transmitting datawords via a multicarrier signal to a receiver and a method of transmitting datawords via a multicarrier signal from a transmitter to a receiver.
A transmission system according to the preamble is known from WO 98/10567. Multicarrier transmission systems, such as systems employing Orthogonal Frequency Domain Multiplexing (OFDM), enable the transmission of data at a relatively high rate. OFDM, also known as Multicarrier Modulation (MCM) or Discrete MultiTone modulation (DMT), is a technique by which data is transmitted by modulating several low bit rate carriers in parallel, rather than one high bit rate carrier. OFDM is spectrally efficient, and has been shown to be effective for, for example, high performance digital radio links. OFDM is also being used in Digital Subscriber Line systems, e.g. ADSL- or HDSL-systems.
An important characteristic of a multicarrier signal is the crest factor, defined as the ratio of the peak value of an AC waveform to its Root Mean Square (RMS) value. In an OFDM system the crest factor can be high since it is possible for the signals on each of the carriers to be in phase, which gives rise to a relatively high peak value, but on average the phases will be randomly distributed, which gives rise to a much lower mean value. For example, in a 16 carrier OFDM system the peak power can be 16 times the mean transmission power.
As most amplifiers have a limited linear range, the (average) power of the OFDM signal must be made much smaller than that of a typical single-carrier signal in order to prevent clipping. Usually a so-called back-off of 10-12 dB is applied. Clipping of the OFDM signal results in a decrease of the signal itself, and the distortion manifests itself as additive noise.
In the known transmission system the crest factor is reduced in two steps: first a number of alternative sequences are generated for each dataword, and second the alternative sequence with the lowest peak power value is selected for transmission to the receiver. The alternative sequences are generated by multiplying carrier values, i.e. the multicarrier representation of the datawords, with arbitrarily selected modification vectors.